In-class group activity 16: Fission
Some nuclear reactions do not occur spontaneously, but require external sources of energy, in the form of "collisions" with outside particles. "Activation energy" has to be provided before a much larger amount of "reaction energy" is released. Nuclear fission can be "activated" when a slow neutron collides with a fissionable nucleus.
Use an on-line
simulation from the University of Colorado PhET group to explore nuclear fission.
Link to the simulation http://phet.colorado.edu/en/simulation/nuclear-fission. Click "Run Now!".
Open a Microsoft Word document to keep a log of your procedures, results and discussions.
Click the Fission: One Nucleus tab.
Before the gun is fired, is the material stable?
What type of "bullet" does the gun fire?
What happens to the nucleus when it is hit?
Click the Chain Reaction tab.
Add some uranium-238. Is uranium-238 “fissionable”? How does firing the gun on a uranium-238 nucleus change it? (Note you can aim the gun.)
Reset the sim and add ~50 fissionable uranium-235 nuclei. Fire the gun. Describe what happens and why it happens.
Naturally occurring levels of the U-235 isotope are about 0.72%, with the majority being U-238. Round the level up to 1% U-235 (one atom of U-235 and 99 atoms of U-238). Use the simulation to find out if naturally derived uranium is able to start a chain reaction,
Use the simulation to find a minimum ratio of U-235 to U-238 that can start a chain reaction? Compare your mixture to “weapons-grade” enriched uranium (about 80%-85% U-235).
Use the simulation to make a nuclear weapon. What conditions are needed? (Check the box “containment vessel”, and determine the level of enrichment needed.)
Click the Nuclear Reactor tab.
What is needed to start the nuclear reactor?
What does adjusting the control rods accomplish?
Without the control rods in position, what happens?
Watch the linked video. Is this a good analogies of a nuclear chain reaction? If mousetraps and Ping-Pong balls are used to illustrate a fission chain reaction, what do each represent?